This invention relates to the field of piezoresistive transducers, and more specifically to pressure transducers having piezoresistors in a bridge arrangement.
Pressure transducers which used piezoresistors in a bridge arrangement are well known in the art. Generally, a silicon substrate was formed and epitaxial layer was grown on the substrate. Resistors were then diffused into the epitaxial layer. Lastly, a portion of the substrate was etched away to leave a thin diaphragm, formed from the epitaxial layer, and a supporting rim. The piezoresistors were located such as to be in the area unsupported by the rim. The end result was a pressure transducer.
To use the transducer, a first and second pressures were exposed to the diaphragm, one on each side. The diaphragm would then deflect in an amount proportional to the difference between the first and second pressures. The deflection, in turn, caused bending of the piezoresistors. This bending of the piezoresistors caused changes in their resistance value, which was reflected as a change in the output voltage signal of the resistive bridge.
Three problems existed with the pressure transducers of the prior art. First, alignment of required regions on the diaphragm was difficult. Resistors needed to be placed at exact locations on the diaphragm. Misalignment of the resistors affected the relationship between the pressure being measured and the accuracy of the output signal from the transducer.
Second, the sensitivity of the prior art pressure transducer suffered because the diaphragm was not effective in transferring the full amount of the deflection to the region of the piezoresistors. Thus, small deflections of the diaphragm did not cause a change in the resistance of the piezoresistors and was therefore not reflected in the output voltage signal of the resistive bridge.
The third problem was that the linearity of the output voltage signal was poor due to pressure variable stretching of the diaphragm in the vicinity of the piezoresistors. This stretching caused a loss of transfer of the deflection to the piezoresistors which thereby caused the non-linearity in the output voltage signal.
Thus, it is an object of the present invention to improve the relationship of the pressure being sensed to the accuracy of the output signal. It is another object of the present invention to improve the sensitivity of piezoresistive pressure transducers. It is yet a further object of the present invention to improve the linearity of piezoresistive pressure transducers.